The present invention relates, in general, to the noninvasive monitoring of respiration rate based on optical (visible and/or non-visible spectrum) signals and, in particular, to monitoring respiration based on the processing of received optical signals to identify heart rate variability associated with respiration. The invention can be readily implemented in connection with pulse oximetry instruments so as to expand the utility of such instruments.
Photoplethysmography relates to the use of optical signals transmitted through or reflected by a patient""s blood, e.g., arterial blood or perfused tissue, for monitoring a physiological parameter of a patient. Such monitoring is possible because the optical signal is modulated by interaction with the patient""s blood. That is, interaction with the patient""s blood generally involving a wavelength and/or time dependent attenuation due to absorption, reflection and/or diffusion, imparts characteristics to the transmitted signal that can be analyzed to yield information regarding the physiological parameter of interest. Such monitoring of patients is highly desirable because it is noninvasive, typically yields substantially instantaneous and accurate results, and utilizes minimal medical resources, thereby proving to be cost effective.
A common type of photoplethysmographic instrument is the pulse oximeter. Pulse oximeters determine an oxygen saturation level of a patient""s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient""s tissue. In particular, pulse oximeters generally include a probe for attaching to a patient""s appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, through the patient""s appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient""s pulse cycle, information can be obtained regarding blood oxygen saturation.
The algorithms for determining blood oxygen saturation related values are normally implemented in a digital processing unit. Accordingly, one or more analog to digital (A/D) converters are generally interposed between the detector and the digital processing unit. Depending on the specific system architecture employed, a single multi-channel digital signal may be received by the digital processing unit or separate digital signals for each channel may be received. In the former case, the digital processing unit may be used to separate the received signal into separate channel components. Thus, in either case, the digital processing unit processes digital information representing each of the channels.
Such digital information defines input photoplethysmographic signals or xe2x80x9cpleths.xe2x80x9d These pleths generally contain two components. The first component of interest is a low frequency or substantially invariant component in relation to the time increments considered for blood oxygen saturation calculations, sometimes termed the xe2x80x9cDC component,xe2x80x9d which generally corresponds to the attenuation related to the non-pulsatile volume of the perfused tissue and other matter that affects the transmitted plethysmographic signal. The second component, sometimes termed the xe2x80x9cAC component,xe2x80x9d generally corresponds to the change in attenuation due to the pulsation of the blood. In general, the AC component represents a varying waveform which corresponds in frequency to that of the heartbeat. In contrast, the DC component is a more steady baseline component, since the effective volume of the tissue under investigation varies little or at a low frequency if the variations caused by the pulsation of the heart are excluded from consideration.
Pulse oximeters typically provide as outputs blood oxygen saturation values and, sometimes, a heart rate and a graphical representation of a pulsatile waveform. The information for generating each of these outputs is generally obtained from the AC component of the pleth. In this regard, some pulse oximeters attempt to filter the DC component from the pleth, e.g., in order to provide a better digitized AC component waveform. Other pulse oximeters may measure and use the DC component, e.g., to normalize measured differential values obtained from the AC component or to provide measurements relevant to motion or other noise corrections. Generally, though, conventional pulse oximeters do not monitor variations in the DC component of a pleth or pleths to obtain physiological parameter information in addition to the outputs noted above. Although it has been proposed to use pulse oximeters to monitor other parameters including respiration rate, it is apparent that such proposed uses have not gained general commercial acceptance.
The present invention is directed to monitoring patient respiration based on a pleth signal. The invention thus provides important diagnostic or monitoring information noninvasively. Moreover, various aspects of the invention can be implemented using one or more channels and/or other components of a conventional pulse oximeter, thereby providing additional functionality to instruments that are widely available and trusted, as well as providing access to important information for treatment of patients on a cost-effective basis.
In accordance with one aspect of the present invention, a pleth signal is analyzed to identify a heart rate variability parameter associated with respiration rate. The associated process involves obtaining a pleth signal, processing the pleth signal to obtain heart rate samples, monitoring the heart rate samples to identify a heart rate variability, and determining a respiration rate based on the heart rate variability. It is known that heart rate varies with the respiration cycle, an effect called Respiratory Sinus Arrhythmia. The present invention provides a robust process for monitoring this effect and determining respiration rate based on pleth signals. A novel processor and pulse oximeter incorporating such processing are also provided in accordance with the present invention.
The step of obtaining a pleth signal generally involves receiving a digital signal representative of an optical signal modulated based on interaction with perfused tissue of a patient. Such a signal may be provided using components of a conventional pulse oximeter. Pulse oximeters typically transmit red and infrared signals, thereby yielding red and infrared pleths. Either or both of these pleths may be utilized in accordance with the present invention. In particular, each of these pleths generally has a fundamental frequency corresponding to the patient""s heart rate. Accordingly, either pleth can be used to yield the desired heart rate information. In general, for normally oxygenated patients, the infrared channel typically has the stronger pleth waveform and may be preferred for heart rate calculations. For poorly oxygenated patients, the red pleth may be preferred. In many cases, a combination of the two signals may provide a better waveform for heart rate analysis than either signal alone.
The pleth may be processed to obtain heart rate samples in a variety of ways. As noted above, the pleth is generally a periodic signal having a fundamental frequency corresponding to the patient""s heart rate. Accordingly, heart rate may be determined by performing peak-to-peak measurements on the pleth to determine the pulse period and, hence, pulse frequency. For example, such maxima may be obtained by identifying a change in sign of differential values between successive samples or groups of samples along the pleth or of a function fitted to the pleth. Alternatively, other points on the waveform, such as nominal zero (or average pleth value) crossings may be monitored. Such zero crossings would be expected to have a frequency of twice the heart rate. Such period measurements can be complicated due to the typically noisy waveform of the pleths. Accordingly, multiple waveforms may be utilized.
Additionally, the heart rate calculations may be performed in the frequency domain. In this regard, a processor may be configured to obtain a Fourier transform of the pleth. Once the Fourier transform is obtained, the pulse rate can be identified as the fundamental frequency of the pleth corresponding to the patient""s heart rate. In any case, once the heart rate is determined, it can be monitored to identify low frequency variations associated with respiration. In particular, oscillatory variations having a frequency of between about 0.15 and 0.5 Hz and, especially, between about 0.2 and 0.4 Hz, are indicative of respiration rate. This range may be expanded to 0-5 Hz to accommodate the higher respiration rates of newborns.
One or more filters may be used in determining respiration rate information based on a pleth signal in accordance with the present invention. In this regard, an adaptive filter may be used to track the fundamental frequency of the pleth and, hence, the patient""s pulse rate. For example, such a filter may function as a narrow band pass filter having a band pass that is centered on the fundamental frequency of the pleth. The transfer function of the filter may be varied, e.g., based on analysis of successive waveforms, to track the changing fundamental frequency. The filter or associated logic may thus be adapted to output a time series of pulse rate values. Such a time series of pulse rate values, whether obtained as an output of an adaptive filter system or otherwise, may be filtered using a static band pass filter having a pass band including the noted frequencies of interest, or using an adaptive filter that tracks a selected spectral peak of the time series to provide an output indicative of respiration rate. Such filtering provides a fast, robust and computationally efficient mechanism for noninvasively monitoring patient respiration based on pleth signals.
The present invention is based in part on a recognition that the pleth signal includes a variety of information in addition to the pulsatile waveform that is generally the focus of plethysmographic processing. In particular, it has been recognized that the pleth signal includes at least three additional or related components: 1) a component related to respiration or the xe2x80x9crespiration wavexe2x80x9d, 2) a low frequency component associated with the autonomic nervous system or vaso motor center, sometimes termed the xe2x80x9cMayer wave,xe2x80x9d and 3) a very low frequency component which is associated with temperature control. Regarding the second of these, the origin and nature of the Mayer wave is not fully settled. For present purposes, the Mayer wave relates to a low frequency variation in blood pressure, heart rate, and/or vaso constriction.
The first two components noted above have particular significance for diagnostic and patient monitoring purposes. In particular, the amplitude and frequency of the Mayer wave are seen to change in connection with hypertension, sudden cardiac death, ventricular tachycardia, coronary artery disease, myocardial infarction, heart failure, diabetes, and autonomic neuropathy and after heart transplantation. Respiration rate is monitored during a variety of medical procedures, for example, as an indication of a patient""s stress levels and to identify patient respiratory distress. It is expected that both the Mayer and respiration waves influence heart rate (and related parameters such as variations in blood pressure and blood volume) by direct influence on the vaso motor center. In the latter case, this is by a xe2x80x9cspilloverxe2x80x9d from the breathing center to the vaso motor center, which increases heart rate during inspiration.
A difficulty associated with obtaining physiological parameter information based on the Mayer wave and the respiration wave relates to distinguishing the effects associated with these waves, particularly in view of the fact that each of these waves can occur within overlapping frequency ranges. In accordance with the present invention, respiration information is obtained by monitoring heart rate variability within a specific frequency band as noted above. In particular, by monitoring in a frequency range having a lower end of preferably at least about 0.15 Hz, for example, 0.15-0.5, interference due to Mayer wave effects can generally be minimized. Still better results may be obtained by monitoring a range between about 0.2-0.4 Hz or, especially, about 0.3 Hz. In the case of tracking respiration rate using an adaptive filter relative to a time series of pulse rate values or a corresponding frequency spectrum, the transfer function may be limited to track the respiration related peak only within these ranges using 0.3 Hz as an initial condition.